Exercise and Gut Flora: Suggestions But No Final Answers

Exercise

By Howell Sasser, PhD
Senior Lecturer, New York Medical College, Valhalla, NY

Dr. Sasser reports no financial relationships relevant to this field of study.

Synopsis: Diversity of gut flora was significantly greater in highly trained athletes than in population controls, even after stratifying on the controls’ body mass index. The athletes’ level of exercise and diet may have been responsible, individually or jointly, for this phenomenon.

Source: Clarke SF, et al. Exercise and associated dietary extremes impact on gut microbial diversity. Gut 2014;63:1913-1920.

A research group in Ireland compared the nature and diversity of gut bacteria in highly trained athletes (professional rugby players) and controls chosen from the general population. All study participants were male, and most were under the age of 40. Participants gave blood and stool samples, completed a detailed food frequency questionnaire, and had several biometric measurements taken. For the presentation of results, controls were divided into two groups by body mass index (BMI) — ≤ 25 kg/m² and > 28 kg/m².

The athletes had significantly (P < 0.009) greater diversity of gut bacterial species as compared with the high-BMI controls. They also had significantly (P < 0.05 to P < 0.009) greater diversity as compared with the low-BMI controls on three of five indices used in the analysis. The athletes showed a significantly different mix of bacterial species as compared with both control groups, although the difference between the athletes and the high-BMI controls was more pronounced. Predictably, the athletes’ diets were higher in average total energy and protein per unit of body mass than either control group (P < 0.05 for each comparison). Higher protein intake was significantly correlated with greater microbial diversity, irrespective of study group (P < 0.0001 to P < 0.03).

Commentary

The results of this study are intriguing, but should be viewed as suggestive rather than conclusive for a number of reasons. First, the design of the study was cross-sectional. Information about diet, exercise, and gut flora was collected at about the same time. Also, the food frequency questionnaire with which diet details were collected covered only the most recent 30 days. This makes it difficult to infer confidently that the observed diet and exercise patterns preceded the observed variations in microbial diversity. It also is unclear what, if any, dose-response relationship there may be, or how quickly a change in diet or exercise patterns might be reflected in the gut flora.

A second issue relates to the nature of the comparisons made in the study. Those in the athlete group were professional rugby players, while those in the comparison groups were selected without respect to their usual level of exercise. A comparison of creatine kinase levels in the three groups (used as a marker of training intensity) showed that the athletes were significantly higher (P < 0.0001) as compared with each control group. While the results of this study may be valid, they seem unlikely to be readily adaptable to recommendations in general medical practice. Advice that a patient begin what the authors of this study themselves describe as “extreme exercise” is unlikely to be heeded.

A third issue is the lack of adjusted analyses. The authors note that extremes of both diet and exercise may affect the composition of the gut flora. They properly collect data on both, but do not report analyses of either potential explanatory factor after controlling for the other. Since they report that the athletes’ diets were different in various respects from those of the other study participants, this is an important unanswered question.

Despite these limitations, this adds to the growing evidence chain that leads from behavioral factors to the gut flora to immune function and other outcomes. There is good evidence that the gut flora are affected by diet and that this effect varies even with seasonal changes in diet.^1,2 There is less clear evidence about the role of exercise, so this paper is a welcome addition in that respect. Exercise, in moderate quantities, has long been linked with improved general health, including fewer sick days.³ However, intense exercise has been shown to be associated with transient impairments of immune function, perhaps through the suppression of neutrophils. In turn, the role of the gut microbiota in affecting (or even “regulating”) immune function has been the subject of considerable research in both animal and human models.^4,5 The present results suggest a possible line of inquiry that might clarify the role of “upstream” factors.

For the individual clinician and patient, the guidance remains largely the same. Moderate exercise most days of the week appears to produce desirable health effects with minimal risk. A dramatic increase in exercise intensity with the goal of altering immune function may or may not produce the desired result, but may also introduce new risks. The same is true of radical changes in diet. Human physiology and the bacteria in the digestive system have co-evolved and have elaborated to such an extent that short-term changes in behavior may alter their functioning, but these alterations are unlikely to be controllable, much less desirable.

References

1. Scott KP, et al. The influence of diet on the gut microbiota. Pharmacol Res 2013;69:52-60.
2. Davenport ER, et al. Seasonal variation in human gut microbiome composition. PLoS One 2014;9:e90731.
3. Nieman DC, Pedersen BK. Exercise and immune function. Sports Med 1999;27:73-80.
4. Noverr MC, Huffnagle GB. Does the microbiota regulate immune responses outside the gut? Trends Microbiol 2004;12:562-568.
5. Hooper LV, et al. Interactions between the microbiota and the immune system. Science 2012;336:1268-1273.